1. Field of the Invention
The invention provides a class of compounds, pharmaceutical compositions comprising such compounds and methods of using such compounds to treat or prevent leishmaniasis, human African trypanosomiasis and Chagas disease.
2. Background
Leishmaniasis is a disease caused by protozoan parasites that belong to the genus Leishmania and is transmitted by the bite of certain species of sand fly.
Leishmaniasis is mostly a disease of the developing world, and is rarely known in the developed world outside a small number of cases, mostly in instances where troops are stationed away from their home countries. Leishmaniasis can be transmitted in many tropical and subtropical countries, and is found in parts of about 88 countries. Approximately 350 million people live in these areas. The settings in which leishmaniasis is found range from rainforests in Central and South America to deserts in West Asia and the Middle East. It affects as many as 12 million people worldwide, with 1.5-2 million new cases each year. The visceral form of leishmaniasis has an estimated incidence of 500,000 new cases and 60,000 deaths each year. More than 90 percent of the world's cases of visceral leishmaniasis are in India, Bangladesh, Nepal, Sudan, and Brazil. Kabul is estimated as the largest center of cutaneous leishmaniasis in the world, with approximately 67,500 cases as of 2004.
There are four main forms of Leishmaniasis. Cutaneous leishmaniasis is the most common form of leishmaniasis. Visceral leishmaniasis, also called kala-azar, is the most serious form in which the parasites migrate to the vital organs. Visceral leishmaniasis is caused by the parasite Leishmania donovani, and is potentially fatal if untreated. Currently, no vaccines are in routine use.
The two main therapies for visceral leishmaniasis are the antimony derivatives sodium stibogluconate (Pentostam®) and meglumine antimoniate (Glucantim®). Sodium stibogluconate has been used for about 70 years and resistance to this drug is a growing problem. In addition, the treatment is relatively long and painful, and can cause undesirable side effects. Amphotericin (AmBisome) is now the treatment of choice. Miltefosine (Impavido) and paromomycin are the other treatment alternatives. These drugs are known to produce a definitive cure in >90% of patients. Amphotericin (AmBisome) is expansive and has to be given intravenously; it is not affordable to most patients affected. Paromomycin, although affordable, requires intramuscular injections for 3 weeks; compliance is a major issue. Miltefosine is an oral drug and has shown to be more effective and better tolerated than other drugs. However, there are problems associated with the use of miltefosine that arise from its teratogenicity and pharmacokinetics. Miltefosine was shown to be much slower eliminated from the body and was still detectable five months after the end of treatment. The presence of subtherapeutic miltefosine concentrations in the blood beyond five months after treatment might contribute to the selection of resistant parasites and, moreover, the measures for preventing the teratogenic risks of miltefosine must be reconsidered. This led to some reluctance to taking Miltefosine by affected populations.
The Drugs for Neglected Diseases Initiative is actively facilitating the search for novel therapeutics. Our invention meets that needs.
Human African trypanosomiasis (HAT), also known as African sleeping sickness, is a vector-borne parasitic disease caused by the protozoa Trypanosoma brucei. There are two subspecies that infect humans, T.b. gambiense and T.b. rhodesiense, with the former accounting for over 95% of reported cases and the latter accounting for the remaining reported cases. The parasites are transmitted to humans by tsetse fly (Glossina genus) bites which have acquired their infection from human beings or from animals harboring the human pathogenic parasites.
The disease has been recorded as occurring in 36 countries, all in subtropical and equatorial Africa. It is endemic in southeast Uganda and western Kenya. In 1995, the WHO estimated that 300,000 people were afflicted with the disease. In its 2001 report, the WHO set the figure of people at risk of infection at 60 million, of which only 4 to 5 million had access to any kind of medical monitoring. In 2006, the WHO estimated that about 70,000 people could have the disease, and many cases are believed to go unreported. About 48,000 people died of sleeping sickness in 2008. Public health efforts in prevention and the eradication of the tsetse fly population have proven to be successful in controlling the spread of the disease; under 10,000 new cases were reported in 2009 according to WHO figures, which represents a huge decrease from the estimated 300,000 new cases in 1998.
African trypanosomiasis symptoms occur in two stages. In the first stage, known as the haemolymphatic phase, the trypanosomes multiply in subcutaneous tissues, blood and lymph. The haemolymphatic phase is characterized by bouts of fever, headaches, joint pains and itching. In the second stage, the neurological phase, the parasites cross the blood-brain barrier to infect the central nervous system. It is in this stage when more obvious signs and symptoms of the disease appear: changes of behaviour, confusion, sensory disturbances and poor coordination. Disturbance of the sleep cycle, which gives the disease its name, is an important feature of the second stage of the disease. Without treatment, the disease is invariably fatal, with progressive mental deterioration leading to coma, systemic organ failure, and death.
Four drugs are registered for the treatment of sleeping sickness. The protocol depends on the stage of the disease. The current standard treatment for first-stage disease is intravenous or intramuscular pentamidine (for T.b. gambiense), or intravenous suramin (for T.b. rhodesiense). The current standard treatment for second-stage disease is: Intravenous melarsoprol, or interavenous melarsoprol in combination with oral nifurtimox, intravenous eflornithine only or eflornithine in combination with nifurtimox. All of the drugs have undesirable or sometime serious side effects. For example, 3-10% of patients those injected with Melarsoprol (Arsobal), an organoarsenical, developed reactive encephalopathy (convulsions, progressive coma, or psychotic reactions), and 10-70% of such cases result in death.
Chagas disease, also called American trypanosomiasis, is a tropical parasitic disease caused by the flagellate protozoan Trypanosoma cruzi. T. cruzi is commonly transmitted to humans and other mammals by the blood-sucking “kissing bugs” of the subfamily Triatominae (family Reduviidae).
Chagas disease is contracted primarily in the Americas. It is endemic in twenty one Cental and Latin American countries; particularly in poor, rural areas of Mexico, Central America, and South America. Large-scale population movements from rural to urban areas of Latin America and to other regions of the world have increased the geographic distribution of Chagas disease, and cases have been noted in many countries, particularly in Europe. Although there are triatomine bugs in the U.S., only very rarely vectorborne cases of Chagas disease have been documented.
Each year, an estimated 10 to 15 million people across the world are infected with Chagas disease, most of whom do not know they are infected. Every year, 14,000 people die as a consequence of the disease. In Central and South America, Chagas kills more people than any other parasite-borne disease, including malaria. By applying published seroprevalence figures to immigrant populations, CDC estimates that more than 300,000 persons with Trypanosoma cruzi infection live in the United States. Most people with Chagas disease in the United States acquired their infections in endemic countries.
Chagas disease has an acute and a chronic phase. If untreated, infection is lifelong.
Acute Chagas disease occurs immediately after infection, may last up to a few weeks or months, and parasites may be found in the circulating blood. Infection may be mild or asymptomatic. There may be fever or swelling around the site of inoculation (where the parasite entered into the skin or mucous membrane). Rarely, acute infection may result in severe inflammation of the heart muscle or the brain and lining around the brain. The initial acute phase is responsive to antiparasitic treatments, with 60-90% cure rates. Following the acute phase, most infected people enter into a prolonged asymptomatic form of disease (called “chronic indeterminate”) during which few or no parasites are found in the blood. During this time, most people are unaware of their infection. Many people may remain asymptomatic for life and never develop Chagas-related symptoms.
However, an estimated 20-30% of infected people will develop debilitating and sometimes life-threatening medical problems over the course of their lives.
The symptoms of Chagas disease vary over the course of an infection. In the early, acute stage, symptoms are mild and usually produce no more than local swelling at the site of infection. The initial acute phase is responsive to antiparasitic treatments, with 60-90% cure rates. After 4-8 weeks, individuals with active infections enter the chronic phase of Chagas disease that is asymptomatic for 60-80% of chronically infected individuals through their lifetime.
There is no vaccine against Chagas disease. Treatment for Chagas disease focuses on killing the parasite and managing signs and symptoms.
During the acute phase of Chagas disease, the drugs currently available for treatment are benznidazole and nifurtimox. Once Chagas disease reaches the chronic phase, medications aren't effective for curing the disease. Instead, treatment depends on the specific signs and symptoms. However, problems with these current therapies include their diverse side effects, the length of treatment, and the requirement for medical supervision during treatment. Resistance to the two frontline drugs has already occurred. The antifungal agent Amphotericin b has been proposed as a second-line drug, but this drug is costly and relatively toxic.
In view of the foregoing, it is desirable to develop novel compounds as antiparasitic agents.